Method for depositing solder material on an electronic component part

ABSTRACT

A method for accurately depositing a required volume of solder material on a specific area of a lead frame, substrate or other part ( 4 ) of an electronic component to be bonded by reflow of solder material to another part into a reliable, void-free connection during a subsequent assembly step comprises the following steps. Minute particles ( 3 ) of solder material whose cumulative volume corresponds to the total volume to be deposited are loaded into a cavity ( 2 ) cut into a fixture ( 1 ) made from a material such as graphite. The cavity delineates the specific area of deposit. The part ( 4 ) is then laid upon the fixture and immobilized thereon by a cover ( 7 ) made from a material such as graphite. The fixture and its enclosed part are then subjected to solder material melting temperature under a controlled atmosphere in a furnace. The cavity is patterned and dimensioned to accommodate the right number of uniformly dimensioned particles necessary to precisely create the desired deposit of solder material.

PRIORITY APPLICATION

This is continuation of U.S. patent application Ser. No. 12/418,845,filed Apr. 6, 2009, a continuation of U.S. patent application Ser. No.12/159,682, filed Jun. 30, 2008, a 371 of PCT/US06/49669 filed Dec. 29,2006 which is a continuation-in-part of U.S. patent application Ser. No.11/323,444 filed Dec. 12, 2005 and a continuation-in-part of U.S. patentapplication Ser. No. 11/523,895 filed Sep. 19, 2006.

FIELD OF THE INVENTION

This invention relates to microelectronic assemblies and packaging, andmore particularly to the deposition of soldered strips or other shapedpatches on electronic component parts such as lead frames, package lids,or substrates for later reflow and connection.

BACKGROUND

The extensive miniaturization of electronic circuits and their packagingrequires the accurate deposition of minute, accurate quantity dabs,lines, or other shaped patches of solder over delineated areas of acomponent surface for future connecting of leads, lids and other partsby reflow.

The solder must be applied in controlled quantity and precisely ontarget in order to avoid bridging or unwanted gaps with other solderedpoints or circuit parts.

In the prior art, stamped soldered preforms are tack-welded to, orsolder strips are laid on the electronic assembly or package in order tohold the solder in place for later remelting.

This invention results from attempts to devise a more precise method fordepositing minute, accurate amounts of solder in precise locationswithout portions coming out of the demarcated area.

SUMMARY

The instant invention provides a method for more accurately depositing aspecified amount of solder material on a precisely delineated area on asurface of an electronic component part that will be subsequentlysubjected to reflow.

In some embodiments the method can used in order to establish a reliableand void-free connection with another component part. In someembodiments the component part may be a lead frame, package lid,substrate or other part. In some embodiments the volume of requiredsolder material is calculated as the product of the area to be coveredby the solder material times the desired height of the solder patch orstrip. In some embodiments this volume is used to calculate the numberor amount of solder material particles which are loaded into a cavitycut in the exposed surface of a fixture made of high density graphite orother crucible-type material having a melting temperature substantiallyhigher than the melting point of the solder material. In someembodiments the delineated area of the part upon which the soldermaterial is to be deposited can be positioned against the exposedsurface of the fixture, and the combined fixture and part are exposed toa temperature at least as high as the melting temperature of the soldermaterial in a batch or belt furnace under a controlled atmosphere.

In some embodiments the top surface of the fixture can be shaped anddimensioned for intimate contact with the solder material deposit area.In some embodiments the cavity carved into said exposed surface can beshaped to be congruent with the deposit area when the part is positionedagainst the fixture with the particles in contact with said area.

In some embodiments the part can be secured upon the fixture with acover of the same high melting point material.

In some embodiments the particles can be laid in one or more rows intothe cavity. In some embodiments the particles can be selected to havethe same calculated, uniform range of dimensions and can be symmetrical,spherical, cylindrical or other shapes.

In some embodiments the cumulative volume of all the particles is equalto the total metered volume of solder material to be deposited.

In some embodiments the cavity has a constant depth which is greaterthan the diameter or size of the particles. In some embodiments thefixture and part are inverted prior to introduction into the furnace sothat the particles drop into contact with the area of deposit.

In other embodiments the cavity has a depth that is lesser than thediameter or other appropriate size dimension of the particles andpressure is applied to the cover to hold the particles in position andassure a better adhesion of the solder to the part. In some embodimentsthe fixture may or may not be inverted during heating depending on theflow characteristics of the materials involved.

In some embodiments the cavity has an arcuate bottom whose radius iscommensurate with the radius of the particles.

In some embodiments the bottom of the cavity has a series of sphericaldepressions, each dimensioned to intimately nest a particle of soldermaterial. In some embodiments these depressions are regularly spacedapart at a calculated interval as a function of the total volume ofsolder material to be deposited and the number and size of theparticles.

In some embodiments the cavity is widened and/or segmented to form oneor more variably shaped ditches sized to be filled with an array ofparticles.

Some embodiments provide a method for accurately depositing a meteredvolume of solder material of a given melting point on a delineated areaof an electronic component part, said method comprising the steps of:providing a fixture having a top surface shaped and dimensioned forintimate contact with said area, and a melting temperature substantiallyhigher than said melting point; carving into said top surface a cavityshaped to be congruent with said area; placing into said cavity a numberof particles of said solder material; positioning said part against saidtop surface and said particles in contact with said area; and exposingsaid fixture and part to a temperature at least equal to said meltingpoint; whereby the material of said particles melts and adheres to saidarea.

In some embodiments the method further comprises securing said part uponsaid fixture with a cover. In some embodiments said fixture is made of amaterial comprising high density graphite. In some embodiments saidsolder material comprises a metal alloy selected from the groupconsisting of gold alloys, tin alloys, lead alloys, copper alloys, andsilver alloys. In some embodiments said solder material comprises ametal alloy selected from the group consisting of AuSn, AuGe, AuSi,AuAgCu, AgCu, and PbSnAg. In some embodiments said delineated areacomprises an electronic package lead frame. In some embodiments saiddelineated area comprises a marginal, peripheral area of amicroelectronic package lid. In some embodiments said cavity issegmented into a plurality of ditches. In some embodiments two of saidplurality of ditches are differently dimensioned. In some embodimentssaid particles are laid in a single row into said cavity. In someembodiments said particles are substantially uniform. In someembodiments said particles are laid in a plurality of rows into saidcavity. In some embodiments said particles are symmetrical, and havecalculated dimensions. In some embodiments said particles are spherical,and have a calculated diameter and radius. In some embodiments thecumulative volume of said particles is equal to said metered volume. Insome embodiments said cavity has a constant depth greater than saiddiameter. In some embodiments said step of positioning comprisesinverting said fixture and part. In some embodiments said cavity has aconstant depth lesser than said diameter. In some embodiments the methodfurther comprises pressing said part against said fixture. In someembodiments said cavity has an arcuate bottom of a radius commensuratewith the radius of said particles. In some embodiments said cavity has aseries of spaced-apart bottom separators dimensioned to intimately nestsaid particles. In some embodiments said separators are regularlyspaced-apart at a calculated interval.

Some embodiments provide a method for accurately depositing a volume ofsolder of a given melting point to a given height on a delineated areaof given superficies on the face of an electronic component, said methodcomprising the steps of: providing a slab of material having a bottomsurface shaped and dimensioned for intimate contact with a zone of saidface including said area, and a melting temperature substantially higherthan said melting point; drilling through said slab at least one boresubstantially perpendicular to said bottom surface and having a loweropening in said bottom surface falling within said area when said bottomsurface is in contact with said zone; carving into said bottom surfaceand around said bottom opening, a cavity at least as deep as said heightand shaped to be congruent with said delineated area; intimatelycontacting said face with said slab's bottom surface; inserting intosaid bore, a particle of said solder, said particle having a volumesubstantially equal to a product of said superficies times said height;and exposing said component and slab to a temperature at least equal tosaid melting point; whereby said particle of solder melts, flows anddeposits accurately over said delineated area.

In some embodiments said delineated area is elongated about an axis;said step of drilling comprises drilling a plurality of said boresleading to spaced-apart openings along a line parallel to said axiswithin said cavity; and said step of inserting comprises inserting atleast one of said particles in each of said bores, said particles havinga cumulative volume equal to said product. In some embodiments saidmaterial comprises high density graphite. In some embodiments saidsolder comprise a gold alloy. In some embodiments said alloy is Au/Sn.In some embodiments said delineated area comprises an electronic packagelead attachment area on a lead frame. In some embodiments saiddelineated area comprises a marginal, peripheral area on an electronicpackage lid.

Some embodiments provide a device for accurately depositing a volume ofsolder to a given height on a limited area on the face of an electroniccomponent which comprises: a slab of material having a top surface, anda bottom surface shaped and dimensioned for conformingly resting upon azone of said face including said area; said slab having at least onebore having a upper opening in said top surface and a lower opening insaid bottom surface, said lower opening being positioned above said areawhen said bottom surface rests upon said zone; and said slab furtherhaving a cavity in said bottom surface and around said bottom opening,said cavity having a depth greater than said height, and being shaped tocongruently fit over said limited area. In some embodiments the devicefurther comprise a plurality of said bores having lower openingspositioned at regularly spaced-apart locations along a line. In someembodiments said limited area comprises at least one lead connectionspot on an electronic package lead frame. In some embodiments saidlimited area comprises a marginal, peripheral area on an electronicpackage lid. In some embodiments the device further comprises a smallvolume of solder inserted in each of said bores. In some embodimentssaid volume is obtained by dividing the amount of solder to be depositedby the number of bores. In some embodiments each of said volume consistsof a particle of solder. In some embodiments said particle of solder hasa shape selected from the group consisting of a sphere, a cylinder, anda quadranglarly sided shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of the top surface of the fixture.

FIG. 2 is a diagrammatical cross-section of the cavity in a firstembodiment of the fixture.

FIG. 3 is a partial diagrammatical cross-sectional view of the firstembodiment of the fixture placed in a furnace.

FIG. 4 is a diagrammatical cross-section of a second embodiment of thecavity section.

FIG. 5 is a diagrammatical cross-sectional view of the cavity section ina third embodiment of the fixture.

FIG. 6 is a perspective partial view of a fourth embodiment of thecavity section.

FIG. 7 is a perspective partial view of a fifth embodiment of the cavitysection.

FIG. 8 is a bottom plan view of a template according to the invention;

FIG. 9 is a cross-sectional view taken along line 9-9 of FIG. 8;

FIG. 10 is a cross-sectional view taken along line 10-10 of FIG. 8;

FIG. 11 is a top plan view of the template applied to a lead frame;

FIG. 12 is a bottom plan view of a template for an electronic packagelid; and

FIG. 13 is a cross-sectional view of a package lid, template and framingfixture assembly.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Referring now to the drawing, there is shown in FIG. 1 the top surfaceof a fixture 1 according to the invention. The fixture 1 is particularlyadapted for depositing a strip of solder in a marginal area around theperiphery of the lid of a microcircuit component. A cavity in the formof an elongate channel 2 carved in the top surface of the fixture isshaped and dimensioned to be congruent to the area targeted to receive adeposit of solder material. The cavity 2 has been filled with a numberof uniformly dimensioned particles of solder material selected in thiscase to be in the shape of spheres 3. The cumulative volume of all thespheres contained within the cavity corresponds to the total volume ofsolder material to be deposited. Using uniformly dimensioned particlesallows for precise control of the total volume of solder material to bedeposited.

As more specifically illustrated in FIG. 2, the cavity 2 has a constantdepth P that is greater than the diameter D of the spheres 3. The partupon which the solder material is to be deposited, in this case amicroelectronic package lid 4, is held in a depression 5 in the topsurface of the fixture that is shaped and dimensioned for intimatecontact with the lower surface 6 of the lid. A cover 7 and releasablefastening means such as a clamp C is used to tightly secure the lid part4 to the fixture 1.

The combined fixture, lid and cover is inverted prior to introducing itinto a furnace 8 as shown in FIG. 3. The spheres 3 have dropped withinthe cavity 2 and come in contact with the lid 4. When the combination issubjected to a temperature at least as high as the melting temperatureof the solder material melts and adheres to the under surface 6 of thelid.

In the second embodiment of the cavity illustrated in FIG. 4, the depthQ of the cavity 2 is lesser than the diameter D of the spheres 3. Inthis case, it is not absolutely necessary, but will typically berecommendable to invert the combination fixture lid and cover prior tointroduction into the melting furnace. The decision whether or not toinvert will depend on the characteristics of the materials involvedincluding wettability of the lid by the chosen solder material. Apressure A is preferably applied to the lid to force the spheres inconstant contact with the under surface 6 of the lid, and assure abetter adhesion of the melted solder material to the lid.

As shown in FIG. 5, the bottom 9 of the cavity 2 is preferably arcuatewith a radius commensurate with the radius of the spheres 3.

In a fourth embodiment of the cavity illustrated in FIG. 6, a series ofholes 10 are carved at regularly spaced intervals I in the bottom of thecavity 2 to act as a localizer or separator for spaced apart particles.In this embodiment, each hole 10 is spherically concave and has the sameradius as the spheres in order to intimately nest one of the spheres 3.It is noted that the hole radius can be slightly smaller than thespheres and still provide localization. Although the holes are shownhaving a partially spherical shape, other shapes either concave orconvex which allow nesting or other localization of the spheres may beacceptable. The practicality of the selected shape is generallydetermined by which shape is easiest to form during the manufacturing ofthe fixture. Depending on the shape of the hole, the intended shape ofthe solder patch or strip, and the space formed between the holeboundaries and a nested particle, the fixture may require invertingprior to initiating melting.

In a fifth embodiment of the cavity illustrated in FIG. 7, the cavity issegmented into one or more ditches 15 having a specified shape anddimensioning. The size and shape of the particles is selected to form anarray of particles 16 nested within each ditch and to provide therequired amount of solder material. For example, in a ditch having arounded corner rectangular shape in a top plan view, and having a firstside dimension S and an orthogonal side dimension T, spherical particlesare selected having a diameter so that an integer multiple of thediameter will substantially equal S and another integer multiple willsubstantially equal T. Depending on the acceptable range of volume ofsolder material, the above equalities need not be exact. Further, theditches can be shaped differently from one another depending on theshape and dimensioning of the delineated area to receive the soldermaterial.

In a first step in the disclosed process, the total volume of soldermaterial to be deposited on the delineated area of a component part iscalculated by multiplying the area of deposit by the desired height ofthe solder material strip or patch. The width of the desired strip ordimensions of the desired patch of deposited solder material determinesthe shape and dimensions of the cavity, whether it is segmented intoditches, whether it uses particle separating structures, and the size orsize range of the substantially uniformly dimensioned particles. Theword “substantially” is used because the particles may not need to beexactly uniform but could fall within an acceptable range so that thecompleted strip or patch of solder material is adequately dimensioned.Use of uniformly dimensioned particles provides a means for precisecontrol of the total volume of solder material to be deposited. Forexample, if spherical particles are used, then the maximum diameter D ofthe spheres and the number of spheres is determined by dividing thetotal volume of solder material to be deposited by the volume of eachsphere. Depending upon that number, the spheres may be laid in a singlerow contiguous to each other as shown on FIG. 1, multiple rows as shownin FIG. 7, or may be spaced-apart using separator nesting structures 10shown in the embodiment of the cavity illustrated in FIG. 6. Theinterval I between the nesting structures 10 in a row is determined as afunction of the number of required spheres of solder material and thetotal length of that row in the cavity 2. All of the calculateddimensions are preferably displayed in a spread sheet bearing as entriesthe various determinant parameters such as the width and length of thedelineated area to be covered by the solder material, the desiredthickness of the material, the length and width of the cavity etc.according to well-known techniques for convenient use by themanufacturer.

In the embodiments of the cavity illustrated in FIGS. 2-5, the loadingof the spheres into the fixture can be done by pouring the spheres intothe depression 5 in the top surface of the fixture that is shaped andconfigured to receive the component part and may also include a gate forexcess sphere removal. Once the cavity has been filled, the excessspheres can be swept away through the gate, not shown on the drawing,practiced in the periphery of the fixture. It is important to note thatthe loading of particles having a shape other than spheres can be donethrough pouring so long as the groups of particles are flowable. Suchflowable groups of particles can be in the form of nanoparticles orflowable metal powders made from techniques well known in theelectronics industry. Optionally, the fixture may be vibrated to avoidclumping of particles and encourage packed nesting.

The pressure A applied on the lid in the second embodiment of the cavityillustrated in FIG. 4, is preferably imparted by a spring pressure clampin the range of 750 to 1,000 milligrams (about 1.5 to 2 lbs.).

The oven is purged of all air and filled with a controlled atmosphereconducive to reflow of the solder material by being non-oxidizing andcan be for example made of 5 percent hydrogen and 95 percent nitrogen.The temperature of the oven is then raised above the melting point ofthe solder material for a period of time sufficient to melt and join theparticles.

In most applications, the delineated area of the component part uponwhich the solder material must be deposited is coated with gold for bestadhesion with a solder material consisting of a gold alloy such as AuSn,AuGe, AuSi, and AuAgCu, or other alloys such as AgCu, PbSnAg or otherknown solder or brazing material which can include some metal alloys ofgold, tin, lead, copper, and silver. The oven should be raised to atemperature that will melt the solder, such as about 340 degrees Celsiusfor AuSn solder, for approximately 15 minutes in order to assurecomplete melting and joining of the solder material and best adhesion tothe component part.

Referring now to FIGS. 8-11, there is shown an alternate embodiment inwhich a soldering guide or template 101 particularly adapted to deposita series of short strips, patches or spots 102 of solder, illustrated indotted lines in FIG. 11 and oriented along an axis X-X′, upon a leadframe 103. The strips 102 will be later used by reflow process toconnect outside leads to an electronic package. The template 101 isconstituted by a slab of material having a melting temperaturesubstantially higher than melting point of the solder such as a goldalloy solders such Au/Sn solder, preferably, a high density graphite.

The bottom surface 104 of the template is machined to intimately matchand rest upon the zone into which the areas wherein the solder is to bedeposited are located; in this case, the flat top surface 105 of thelead frame. A series of cavities 106 cut into the bottom face 104 of thetemplate are shaped and dimensioned to congruently fit over the areas tobe occupied and delineated by the solder strip 102. The depths D′ of thecavities must be at least as great as the desired height of the solderstrip 102. A series of sets 107 of channels or bores 108 are drilledfrom the upper surface 109 of the template toward the cavities 106. Eachset 107 of bores terminates into a number of lower openings 110regularly spaced-apart in the roof 111 of a cavity along a linesubstantially parallel to the axis X-X′ of the cavities.

It should be understood that if the limited area that will receive thesolder is a small spot, the cavity may be circular and be fed by asingle channel or bore.

As illustrated in FIGS. 12 and 13, a quadrangular template 112 isspecifically intended for use in depositing a continuous bead 113 ofsolder along a marginal, peripheral area of an electronic package lid114 in accordance with the present embodiment. A cavity 115 matching theoutline of the desired bead 113 is cut into the bottom surface 116 ofthe template 112. Bores 17 have their lower openings distributed atregularly spaced-apart locations along the roof of the cavity 115.

FIG. 13 illustrates the positioning of the template 112 over the packagelid 114 within a special jig or fixture 118. A solder particle 119 of adefined volume shape such as a sphere and constituted by a meteredvolume of solder is inserted in the upper opening 120 of each bore.

The whole assembly 121 comprising the fixture 118, the package lid 114and the loaded template 112 is then placed into an oven and exposed to atemperature sufficient to melt the solder under a controlled atmosphere.When melting, the solder from the particles flows into the cavities anddeposits very accurately in the area limited by the overhead cavity 115.The solder is distributed evenly in a continuous strip to a thicknessthat depends upon the size and number of the solder particles 119.

After a cooling period, the template 112 is removed leaving a narrowbead of solder on the periphery of the lid 114 for future reflowattachment when the lid is installed upon an electronic package.

The particles 119 of solder are preferably manufactured according toprocesses well-known to people skilled in the art of metal particlefabrication.

The size of the particles and the diameter of the bores are determinedby first calculating the total volume of the soldered strip or spot,that is, the product of superficies of the targeted area times thedesired height of the solder deposit. This total volume is then dividedby the number of bores leading to the cavity capping that area.

In many electronic package assembly applications where the width of thesoldered traces falls within the range of about 0.3 millimeter (12 mils)and about 0.7 millimeter (28 mils), the particles can be spheres havinga diameter between about 0.35 millimeter (14 mils) and about 0.65millimeter (26 mils) with spacing between the bores of approximately 1millimeter (40 mils). It should be understood that the particles can beprovided in shapes other than spheres, such as cylinders, quadrangularlysided shapes such as blocks, or other readily manufactured shapes havinga defined volume.

EXAMPLE

For the common Au/Sn solder, the electronic component and templateassembly is preferably exposed to a temperature of approximately 360degrees Celsius for thirty minutes in an oven hot zone under anatmosphere of 5% hydrogen and 95% nitrogen with a gas flow of 10 cubicfeet per hour. The assembly is then moved to a cool zone for 20 minutesunder continuous gas flow to prevent oxide formation.

While the preferred embodiments of the invention have been described,modifications can be made and other embodiments may be devised withoutdeparting from the spirit of the invention and the scope of the appendedclaims.

1. A method for accurately depositing a metered volume of soldermaterial of a given melting point on a delineated area of an electroniccomponent part, said method comprising the steps of: providing a fixturehaving a top surface shaped and dimensioned for intimate contact withsaid area, and a melting temperature substantially higher than saidmelting point; carving into said top surface a cavity shaped to becongruent with said area; placing into said cavity a number of particlesof said solder material; positioning said part against said top surfaceand said particles in contact with said area; and exposing said fixtureand part to a temperature at least equal to said melting point; wherebythe material of said particles melts and adheres to said area.
 2. Themethod of claim 1 which further comprises securing said part upon saidfixture with a cover.
 3. The method of claim 1, wherein said fixture ismade of a material comprising high density graphite.
 4. The method ofclaim 1, wherein said solder material comprises a metal alloy selectedfrom the group consisting of gold alloys, tin alloys, lead alloys,copper alloys, and silver alloys.
 5. The method of claim 1, wherein saidsolder material comprises a metal alloy selected from the groupconsisting of AuSn, AuGe, AuSi, AuAgCu, AgCu, and PbSnAg.
 6. The methodof claim 1, wherein said delineated area comprises an electronic packagelead frame.
 7. The method of claim 1, wherein said delineated areacomprises a marginal, peripheral area of a microelectronic package lid.8. The method of claim 1, wherein said cavity is segmented into aplurality of ditches.
 9. The method of claim 8, wherein two of saidplurality of ditches are differently dimensioned.
 10. The method ofclaim 1, wherein said particles are laid in a single row into saidcavity.
 11. The method of claim 1, wherein said particles aresubstantially uniform.
 12. The method of claim 1, wherein said particlesare laid in a plurality of rows into said cavity.
 13. The method ofclaim 1, wherein said particles are symmetrical, and have calculateddimensions.
 14. The method of claim 1, wherein said particles arespherical, and have a calculated diameter and radius.
 15. The method ofclaim 1, wherein the cumulative volume of said particles is equal tosaid metered volume.
 16. The method of claim 14, wherein said cavity hasa constant depth greater than said diameter.
 17. The method of claim 1,wherein said step of positioning comprises inverting said fixture andpart.
 18. The method of claim 14, wherein said cavity has a constantdepth lesser than said diameter.
 19. The method of claim 1, whichfurther comprises pressing said part against said fixture.
 20. Themethod of claim 14, wherein said cavity has an arcuate bottom of aradius commensurate with the radius of said particles.
 21. The method ofclaim 14, wherein said cavity has a series of spaced-apart bottomseparators dimensioned to intimately nest said particles.
 22. The methodof claim 21, wherein said separators are regularly spaced-apart at acalculated interval.